Control of transmission in two-way signaling systems



C. C. TAYLOR July 30, 1940.

2,209,667 CONTROL oF TRANSMISSION IN Two-WAY SIGNALING sYsTEMs lssheefs-sheet 1 Filed May 13. 1939 N @Dx July 30, 1940. c. c. TAYLORCONTROL 0F TRANSMISSION IN TWO-WAY SIGNALING SYSTEMS Filed May 13, 19393 Sheets-Sheet 2 /A/ l/E/v TOR C. C. TA VL OR HTM. KQ@ QY BV cfm( 9WATTORNEY July 30, 1940. c. c. TAYLOR 2,209,667

CONTROL OF TRANSMISSION IN TWO-WAY SIGNALING SYSTEMS' Filed May 15, 19395 Sheets-Sheet 5 r .ov f E wwwmmyle a n Juni N i Il# l v mi /A/l/E/VTORC. C. 7 YLOR .9 v 4e Cfgm ATTO/MEVA Patented July .30, 1940 UNITEDSTAT-Es CONTROL OF TRANSMISSION IN TWO-WAY Y SIGNALING SYSTEMS y CharlesC. Taylor, Manhasset, N. Y., assignor to Bell Telephone Laboratories,

Incorporated,

New AYork, N. Y., a corporation ofNewYork Application May 13, 1939,Serial No. 273,375

9 claims.

The inventionrelates to two-way signal transmission systems andparticularly to the signalcontrolled switching circuits employed Ainsuch systems for directionally controlling transmis- "5 sion whilepreventing singing and suppressing echoes.

An object of the invention is to improve the operation of suchsignal-controlled switching circuits.

l Another object is to directionally control transmission in a two-waytelephone transmission system in such manner as to minimize transmissionlockouts and clipping, and to facilitate break-ins by a listeningsubscriber.

l The circuits of the invention are particularly adapted for use with,although not limited to, a two-way radio telephone system, such as atransoceanic short wave radio telephone circuit, in which diierent radiofrequencies are used in the 2'0 two directions and in which, because ofhigh frequency selectivity, geographic separation, or both, thesuppression of transmission from the local transmitter to the localreceiver at a terminal station is unnecessary.

'2'5 In the circuits of the invention, the transmitting circuit at eachterminal station of a two-way signal transmission system is normallyenabledV and the receiving circuit normally disabled either partially ortotally. When signals are initiated 30 at a terminal station they causea control tone to be transmitted to the distant terminal station aheadof the signals, to control a switching circuit operating to disable thetransmitting circuit and to enable the receiving circuit at thatstation.

35 The receiving station switching circuit has appreciable hang-over inits operation and the arrangement for controlling the tone at thetransmitting station has little or no hang-over in operation so that itoperates and releases rapidly with the rise 46 and fall of the signalamplitudes, providing desirable operation from the standpoint ofclipping,-

lock-out and breaking time. l

In one embodiment of the invention, the tone control arrangement at thetransmitting station 45 comprises a syllabic device connected to thetransmitting circuit at each terminal, which is responsive to appliedvoice signals to increase the sensitivity of two quick responseamplifier-detector circuits respectively bridged across the trans- 50mitting circuit on either side of a delay circuit',

which operate in parallel to control the operation of a tone-controlrelay having little or no hangover in operation. f

A feature of the invention is the use at each 55 terminal station of aswitching circuit responsive to transmitted signals to disable thereceiving control tone-operated switching circuit, at the same station,the rst circuit having a delay in operation and release corresponding tothe transmission time between terminals, thus reducing 5 thetransmission lock-out time.

A second feature is the luse for the receiving circuit disabling meansof a singing suppressor the loss of which is made a function of the gainof a volume control device (Vogad) in the transmitting circuit'at thesame terminal and also depends upon the volume of the line and radionoise present in the receiving circuit. The various features andadvantages of the invention will be better understood from the followingdetailed description thereof when read in conjunction with theaccompanying drawings, in which:

Fig. 1 is a diagram in the form of a single line schematic showing thegeneral arrangement of transmission apparatus at the two terminals ofthe usual two-way radio telephone system;

Fig. 2 and Figs. 3 and 4, combined, show schematically and in partdiagrammatically different modifications of transmission controlapparatus andv circuits embodying the invention which would be used ateach terminal of a system such as shown in Fig. 1.

In the radio telephone system of Fig. 1, each terminal station A and Bcomprises a one-Way transmitting circuit TC including a radiotransmitter RT, and a one-way receiving circuit RC including the radioreceiver RR, coupled in conjugate relation with each other and in energytransmitting relation with a two-way telephone line TL by the usualhybrid coil I-I and associated balancing network N. Each terminalcomprises in addition, signal-controlled switching circuits andapparatus for controlling signal transmission in the transmittingcircuit TC and receiving circuit RC, represented by the box labeledTransmission control. The switching circuits and apparatus which wouldbe used at each terminal station of such a system, embodying differentmodification of the invention, are illustrated in the other gures of thedrawings.

Fig. 2 shows one of two like terminal stations of a two-way radiotelephone system such as shown in Fig. 1 in which the transmission timebetween terminals is represented as T. The ter- 0 minalstation includesa four-wire circuit connecting a telephone line to a radio transmitterand a radio receiver which may be of any wellknown type-or construction.The four-wire circuit vcomprises a transmitting circuit TC leading Thetransmitting circuit TC includes, in order,

connected between the hybrid coil I-I and the radio transmitter I, afixed loss (6 decibels) pad 5, a volume control device 6 known as aVogad (volume operated gain-adjusting device), of any of the well-knowntypes, for example, such as disclosed in Hogg et al. Patent No.1,853,974, Mitchell et al. Patent No. 2,019,577 or Wright et al. PatentNo. 2,043,403, a delay circuit 'I, a transmitting amplier 8 and aprivacy device 9.

The receiving circuit RC includes, in order,

connected between the radio receiver 2 and the hybrid coil H, theband-pass lter I0, a privacy device Il, a receiving amplifier I2, avariable loss network shown within ,the dot-dash box ISI, operating as areceiving singing suppressor, a manually variable loss pad I4 and a xedloss (2 decibels) pad I5.

Connected across transmitting circuit TC at a point between the Vogad iiand the delay circuit 7 is the input of a transmitting switching circuitI 6 comprising an amplifier-detector I 'I,

and the tone control relay I8 and the receiving disabler'relay I9connected in parallel across its output. Connected across the receivingcircuit RC at a point between the radio receiver 2 and the band-passfilter I0 is the input of a receiving switching circuit 20 comprisingthe band-pass lter 2l, the amplifier-detector 22, and the singingsuppressor relay 23 and the echo suppressor relay Ztl connected inseries across the output of the amplier-detector 22.

A source 25 of control or tone energy of substantially constantfrequency, preferably above the voice signal frequency range and whichmay be the same or diirerent frequencies for the tone sources at the twoterminals of the system, is adapted to be connected by operation of thetone control relay I8 under control of amplifierdetector I'I in thetransmitting switching circuit I 6, through the hybrid coil H1 andassociated balancing network N1 across the circuit TC at a point betweenthe radio transmitter I and the privacy device 9. When relay I8operates, the tone energy from the source 25 passes over the circuit TCto the input of the radio transmitter I. Operation of the receivingydisabler relay I9 under the control of the amplifier-detector I'I, witha delay of T with respect to relay I8, will cause the receiving switchcircuit 20 to be disabled at a point between the output of theamplier-detector 22 and the relays 23 and 24.

Operation of the singing suppressor relay 23 under control of theamplifier-detector 22 in the receiving switching circuit 20 will causethe. loss inserted by the receiving singing suppressor device I3 in thecircuit RC to be removed in a manner which will be described later, andoperation of the echo suppressor relay 24 under control of theamplifier-detector 22 will cause the transmitting circuit TC to bedisabled at a point between the loss pad 5 and the Vogad '6, so as tosuppress echoes in the latter circuit, and to effectively disable theVogad 6 and the transmitting switching circuit I6.

The band-pass lter I0 in the receiving circuit RC is adapted to passfrequencies within the range of the voice signals received from theradio receiver 2, and to suppress received waves of the frequencyassigned to the tone source at the distant terminal, whereas theband-pass filter 2I in the receiving switching circuit 20 is arranged totransmit waves of the frequency assigned to the tone source at thedistant terminal while suppressing waves of frequency within the voicesignal frequency range.

The singing suppressor device I3 includes two repeating coils 25 and 26connected in tandem in the receiving circuit RC, a rheostat' 2'Inormally connected in shunt with the receiving circuit RC between thetwo repeating coils through the normally closed contacts of relay 23.The resistance element of rheostat 27 shunted by a series copper-oxiderectifier string 28, is connected in series in the plate-cathode circuitof the Vogad 6 in the transmitting circuit TC, so that the shunt lossinserted in the receiving circuit RC by the singing suppressor device I3depends upon the plate current of the Vogad.

When the terminal is set up for via calls, the relay 30 is operated toshunt a portion of the resistance element of rheostat 2l' by a smallresistance 29 through the contacts of that relay, in order to reduce theshunt resistance across the circuit RC and thus increase the loss of thesinging suppressor device I3 by a suicient amount to preserve thestability of the via connection.

'Ihe (fi-decibel) Xed resistance loss pad 5 and the (2-decibel) xedresistance loss pad I5 which remain in the transmitting circuit TC andthe receiving circuit RC, respectively, for terminal calls, are adaptedto be switched out of these circuits by operation of the relays 3l and32, respectively, in the case of via calls. The windings of relays 3|and 32 are energized in series with the winding of the relay 3d frombattery 33 when a two-wire line section is connected by the plug andjack 4 to the terminal to set up a via circuit, so that the two losspads are removed from the four-wire terminal circuit simultaneously withthe increase in the loss of the singing suppressor device I3.

The operation of a two-way radio telephone system as shown in Fig. lemploying a circuit, such as illustrated in Fig. 2, at each terminalwill now be described.

It will be assumed that the circuits at both of the two terminalstations have been set up for via calls, so that at both stations theloss pads 5 and I5 have been removed respectively from the input of thetransmitting circuit TC and the output of the vreceiving circuit RC ateach terminal, and the receiving singing suppressor I3 in the receivingcircuit RC has been operated to the loss condition required for suchcalls in the manner described above.

The adjustable arm of the rheostat 2 is initially adjusted to determinethe amount of the singing suppressor loss to be used, this adjustmentdepending upon the received noise. For example, with a certain receivednoise it will be determined that a given adjustment of the rheostat 2'Iwill produce sufcient suppressionfloss so that, vfor a minimum returnloss to be expected and low Vogad gain, echo operation of thetransmitting amplifier-detector l? will not occur nor will round tripsinging occur or be within 10 decibels of occurring,l assuming similaradjustments at'the distant terminal. 'This is the mini- 75 vaccessi' mumsinging' suppressor loss, as the amount of this loss when signals arebeing transmitted from theterminal will vary with the Vogad gain in amanner which will be described.

` Now, if the subscriber associated with the twoway line connected tothe west end of the terminal shown in Fig. 2 speaks, assuming there isno incoming speech wave present in the receiving cir-- cuit RC, hisvoice currents impressed on the input ofthe transmitting circuit TC willbe transmitted thereover to the input of the Vogad 6 which regulatesthese voice currents to give the desired constant volume in its output,by adjusting its gain,

An indication of the adjustment of the Vogad gain is` given from thevalue of the plate current flowing in the vario-repeater thereof, thehigher this gain the larger this plate current. Now, if the level of thevoice currents applied to the input of the Vogad are such as to increasethe gain thereof above that value which will require the minimum singingsuppressor loss for which the singing suppressor I3 in the receivingcircuit RC has been initially adjusted as described above, the platecurrent flowing through the copper-oxide rectifier string 28 of thedevice I3 will increase and will reduce the rectifier resistance as thecurrent increases. As this rectifier resistance shunts the resistanceelement of rheostat 21, and thus the'resistance 29 connected across apart of the former resistance, the value of the resistance shunting thecircuit RC is reduced and the singingsuppressor loss therefore isincreased in proportion to the increase in gain of the Vogad.

The voice currents of regulated volume in the output of the Vogad 6 aredivided between the input of the delay circuit 'I in the maintransmission path TC and the input of the transmitting switching circuitI6. Although the primary function of the Vogad 6 is to deliver aconstant signal volume to the radio transmitter I, it has a secondaryfunction which is to effectively vary the sensitivity of thetransmitting Vodas circuit I6 to oompensate for the varying signal inputto that device with strong and weak talkers.

K The voice currents diverted into the switching circuit I6 will beamplified and rectified by the amplifier-detector I'I to cause theoperation of the switching relays I8 and I9, the latter relay with adelay time of T with respect to relay I8, where T is substantially equalto the one-Way transmission time between the two terminal stations.Relay I8 will operate to connect the source' 25 of tone waves to thetransmitting circuit through the hybrid coil H1, and the tone waves fromthat source will pass over the output of the circuit TC to the radiotransmitter I which radiates the tone to the distant terminal. Thespeech waves meanwhile will pass through the delay circuit 'I, amplifier8, the privacy device 9 and the hybrid coil H1 to the radio transmitterI and will also be radiated thereby.

' Operation of the switching relay I9 at a time T after relay I8operates, causes the tone-operated receiving circuit 20 at the sameterminal to be disabled in its output thus preventing subsequent.operation of that switching circuit only after the tone wavestransmitted to the distant terminal have seized control of the switchingcircuits at the distant terminal.

The operation of the distant terminal (not shown) in response to thereceived tone and voice waves, will be explained by referring to theapparatus at that terminal by the same reference numbers as the similarapparatus at the local termnal but followed by a prime mark.

' The' tone waves received by the radio receiver 2 will be detectedytherein and transmitted over the receiving circuit RC'. The band-passfilter I0 in the receiving circuit RC will suppress the tone frequency,but it will be selected by the parallel band-pass filter 2| in thereceiving switching circuit 20 and will cause operation of theamplifier-detector 22 to energize the winding of relays 23 and 24connected to its output. The operation of the relay 24 will disable thetransmitting circuit TC at the receiving terminal in front of the VogadB', and the simultaneous operation of the switching relay 23' will breakthe shunt connection `across the receiving circuit RC betweenthe'repeating coils 25 and 26 through the resistance 29* and a part ofrheostat resistance 2l' in parallel, thereby removing the loss normallyproduced in the receiving circuit RC by the singing suppressor deviceI3.

When the voice signals reach the receiving terminal, they will be pickedup and detected by the radio receiver- 2 and transmitted through thereceivingA circuitV RC". The received voice waves will be passed by theband-pass filter l0', and then will proceed through the privacy deviceII', the amplifier I2', the yreceiving singing suppressor device I3,receiving loss control pad I4 and the hybrid coil H over the associatedtwo-wire line to the listening subscriber. The receiving singingsuppressor device I3' provides little or no loss to the vtransmission ofthe received voice currents due to its `previous operation by thereceived control tone wave as described above to the low loss condition.The volume sent out to the subscriber is adjusted manually by themanually variable. loss pad I 4' to a value considered best forintelligibilitywith the noise or static conditions prevailing. Thisvolume is maintained the same for all connections in this controlcurrent operation for a given noise condition.

When the subscriber associate-d with the transmitting terminal of thesystem ceases talking the transmitting amplifier-detector I'I releasesto release first the switching relay I8 and at a time T-|.05 secondthereafter relay I9 in its output, disconnecting the tone source 25 fromthe transmitting circuit TC and removing the disability in the output ofthe tone-operated receivingcontrol circuit 2li, thereby returning theswitching circuits at the transmitting terminal to the positionindicated in Fig. 2. W'hen the reception of control ton'e ceases at thereceiving terminal, the switching relays 24 and 23 in the tone-operatedreceiving circuit 20' release aftera predetermined hang-over period tobe discussed below to remove the disability in front of the Vogad 6 intheV transmitting circuit TC' and to reconnect the shunt circuit througha portion of the potentiometer resistance 2'I and resistance 29 inparallel across the receiving circuit RC to provide a singing suppressorloss in that circuit the value of which depends on the initialadjustment' of the singing suppressor device I3' and the gain setting atwhich the Vogad-remains.

As explained below, the operate and release orl hang-over'times of theswitching relays at the transmitting and receiving terminals in thesystem described above in accordance with the invention are selected soas to assure optimum conditions for breaking and minimum lockout.

The method of securing optimum breaking time, that is, the shortest timethat one speaker must pause to permit the other talker to reversethe-circuit,` is as follows: i

As the transmitting circuit TC, TC at each of the two terminals isnormally enabled, there is no possibility of speech signal clippingyoccurring in the transmitting branch in consequence of operation of thetransmitting ampliiier-detector. Thus, there is no requirement forappreciably long hang-over on the tone control relay i8, I8 to preventspeech mutilation. The tone control relay, therefore, is provi-ded witha relatively short hang-over (0.06 second, for example) so that it willtend to follow the voice waves closely in on-oif operation, that is, itwill operate and release rapidly with the rise and fall of the voiceamplitudes. With the tone following speech control closely, thetone-operated switching circuit at the receiving end, may be given anappreciable hang-over, (say approximately 0.05 second) without makingthe system have an appreciably longer breaking time than present Vodasswitching circuits employing a' normally disabled transmitting circuitand employing the voice waves themselves for controlling the switchingoperations. The hang-over is added at the contacts of the switchingrelays in the switching circuit, and thus permits the advantage of ahang-over there without the disadvantage cf long hang-over and slowbreaking which results if there is hang-over at both transmitting andreceiving points. A hang-over at the receiving point has the advantagethat it permits a lower sensitivity without clipping Aoccurring in themiddle of phrases, and therefore there is less possibility of falseoperation from noise.

Preferably, the same hang-over (.05 second) should be added to the echosuppressor relay 24 or 24', controlled by the tone-operated receivingswitching circuit, plus an additional hang-over of, say 0.02 second,when the system is :being used for via calls, to suppress static echoeswhich follow cessation of speech signals and which may be delayed by theconnected two-way line. This additional hang-over may be required onlyfor the higher noise conditions.

Transmission lock-out time, that is, the time interval during which theparties associated with the two terminals of the system may each securecontrol of his own terminal but not of the distant terminal if they bothstart talking, normally a time interval equal to the transmission time Tover the radio link plus relay operating time, may be reduced in thefollowing manner.

The operation of the receiving disabler relay i9, i9 at the transmittingstation is Apreferably delayed after operation of the tone control relayl, i8 at the same terminal, for a time interval T where T is equal tothe transmission time of the tone over the system from the transmittingterminal to the distant terminal. This may be obtained by suitabledesign of that relay or of that relay and of a chain of associatedrelays (not shown) by any of the well-known means to make the disablingrelay slow-operating to the required degree. In a typical transoceanicradio telephone system, the time required ior the tone transmitted fromone terminal to complete operation of the singing suppressor relay 23,23 and the receiving echo suppressor relay 24, 24 after arrival at thereceiving terminal i would be in the neighborhood of about 0.005 second,so that with delayed operation each of relays i 9, I 9 a talker securescontrol of his own terminal only .005 second before he secures controlof the distant terminal. There would thus be only a period of about .005second in which lockout may occur. Lock-out time may be reduced to zeroby further delaying the operation of the receiving disabler relay, butthe advantage of doing this is doubtful. Singing may not occur becausethe local signal receiving circuit which is normally disabled Will beprevented from becoming enabled before the distant signal receivingcircuit is enabled.

To reduce the possib-le disturbance due to false operation of thereceiving amplifier-detector by static, because of the lag in theoperation of the receiving disabler relay I9, I9', it may be desirableto give the receiving disabler relay a, hang-over beyond the release ofthe tone contro-l relay I8, I8' (say T-l-.05 secon-d). Although thelock-out condition would not be changed by such a design, some advantagein two-way operation will be secured by this in preventing any falseoperation of the receiving echo suppressor relay 24, 24 during thisperiod in which a signal cannot reach the amplifier-detector from thedistant terminal.

Figs. 3 and 4 placed side by side with Fig, 3 at the left showschematically a two-way radio telephone terminal embodying a modied formof the invention suitable for a radio-telephone circuit in which thesame radio frequency is used in the two directions.

Referring to Figs. 3 and 4 it will be seen that, as in the system ofFig. 2, this terminal includes a transmitting circuit TC leading to aradio transmitter I and a receiving circuit RC leading from a radioreceiver 2, having their respective input and output -coupled insubstantially conjugate relation with each other and in energytransmitting relation with a two-wire line section TL by the hybrid coilH and associated balancing network N. The transmitting circuit TCincludes, in order, connected :between the hybrid coil I-I and the radiotransmitter I, the one-way ampliiier 35, ythe Vogad 39, the delaycircuit 31, the one-way amplifier 38, the two-way or bilateral privacydevice 39 and the band-pass .'lter 40, and the receiving 'circuit RCincludes, in order., connected between the radio receiver 2 and thehybrid coil H, the -band-pass filter 4I, the bilateral privacy device 39and the one-way amplifier 42.

The two-way or bilateral privacy device 39, which may be of the typedisclosed in Fletcher Patent No. 1,573,929, issued February 23, 1926 orin Dickieson Patent No. 2,132,205 issued October 4, 1938, is coupled incommon to the transmitting circuit TCl and the receiving circuit RC byhybrid coils H2 and H3 and associated balancing networks Nz and N3, soas to allow the use of the same privacy device for both directions oftransmission, while maintaining a conjugate relation between the otherportions of the circuits TC and RC.

The transmitting circuit TC is normally enabled, but the receivingcircuit RC is normally disabled at a point 43 near its input betweenband-pass iilter 4I and hybrid coil Hs, and at a point 44 near itsoutput between hybrid coil H2 and amplifier 42.

A source 45 of control waves or tone of .substantially constantfrequency ,f which is preferably outside the voice frequency range andis a different vfrequency for .the tone generators -at the two terminalsof the system, when the same carrier frequency is used for transmissionin the two directions, is adapted to be connected through a band-passlter 4B passing the tone frequency, to the circuit TC at -a .pointvbetween band-pass filter 40 and radio transmitter `I by'operationofOperation of amplifier-detector 48 or amplifier-l the tone control relay41. The band-pass filter 49' is designed to suppress the tone f, and topass waves of frequencies within the voice signal frequency range.

The tone control relay 41 is adapted to be operatively energized eitherby the output currents of the amplifier-detector 48 or ofamplifier-detector 49, which have their outputs connected in parallel tothe winding of that relay and their inputs respectively connectedthrough circuits 50 and 5| to the transmitting circuit TC on the inputand output side of delay circuit 3 1. The input circuit 50 toamplifier-detector 43 includes a manually variable resistance pad 52 anda removable fixed resistance pad 53, and the input circuit 5I toamplifier-detector 49 includes a manually variable resistance pad 54 anda removable fixed resistance pad 55.

Connected across the circuit TC at a point between the output of theVogad 36 and the delay circuit 31 is the input of a circuit, preferablyincluding in common with the circuit 50 the variable resistance pad 52,comprising the syllabic control device 56, and the electromagneticrelays 51 and 58 adapted to be operatively energized in parallel fromthe output of the device 56 when it is operated, to respectively removethe loss pad 53 from the input circuit 50 to the amplifierdetector 48and to remove the loss pad 55 from the input circuit 5| toamplier-detector 49.

The syllabic control device 56 may be any device adapted to operate inresponse to signals applied to its input having the syllabiccharacteristic of speech signals, and to be substantially unresponsiveto comparatively steady noise Waves, due to room and circuit noise. Sucha device may consist of an amplifier-detector circuit tuned to afrequency within the speech frequency range, for example, 1,000 cycles,followed by a lter or tuned circuit adapted to selectively transmit thesyllabic frequencies of speech, for example, a low-pass lter having acut-off at about 22 cycles per second. This circuit preferably wouldhave a high impedance input so that it will insert little loss in thetransmission circuit across which it is bridged. Suitable circuits ofthis type are disclosed, for example, in Nyquist Patent No. 1,749,811,Mathes 1,892,284, Bjornson 1,859,019, Fisher 2,026,305 and Dickieson2,132,205.

Connected across the receiving circuit RC at a point between the radioreceiver 2 and the band-pass filter 4l, which filter is adapted tosuppress waves of the frequency of the tone transmitted from the distantstation while transmitting waves of frequencies within the voice signalband, is the input `of a tone-operated switching circuit 59. ThisVswitching circuit includes a band-pass filter 60 in its input to selectthe tone waves received from the distant terminal while suppressing thevoice frequency signaling band, a variable resistance loss pad 6|,anamplifier-detector 62 and the electromagnetic switching relays 63, 64,65 and 66 adapted to be energized in series from the output ofamplifierdetecto-r 62. The singing suppressor relays 63 and 64, whenenergized, operate to remove the normal disabilities from the circuit RCat the points 43 and 44, respectively. The echo suppressor relays 65and-66 when energized operate to disable the transmitting circuitrespectively at a point 61 between the output of amplifier 35 and theinput to the Vogad 36 and at a point 68 between the hybrid coil H3 andthe band-pass filter 40.

detector 49 in the transmitting switching circuit in response to signalsapplied to their respectivel inputs will operatively energize thereceiving disabler relay 69 in parallel with the tone-control relay 41,but operation of the former relay is delayed with respect to that of thetone-control relay 41 for a time T substantially equal t'o thetransmission time of the tone waves between the two terminal stations ofthe system, by any suitable delay means 10, which may be, for example, aslow-operating relay circuit.

As explained below in connection with the description of operation of atwo-way systemk employing the circuit as shown on Figs. 3 and 4 at eachterminal station, the operate and release or hang-over times of theswitching relays at the two stations are selected in accordance with theinvention to secure very small lock-out time and optimum breakingconditions while taking advantage of hang-over at the receiving stationto aid in control tone operation through fading. i i

Now, if the telephone subscriber associated with thetwo-way line 'I'Lconnected to the west end of the'terminal Station shown in l'igs. 3V and4, speaks, assuming there are no incoming speech waves present in thereceiving circuit RC ofthe terminal, his (wests) voice currents will beimpressed by the hybrid coil H on the input of the transmitting circuitTC and will be amplified by amplier 35 and pass into the Vogad 36. TheVogad 36 will adjust its gain in accordance with the level' of theapplied waves so as to produce the desired constant volume for thevwaves in its outputgwhich will be divided between the delay circuit 31in the main signaling circuit and the common input 50 to the syllabicdevice 55 and the amplifier-detector 48. l

The value of the removableloss pads 53 and 55 in the respective inputstothe amplifier-detectors 48 and 49 are selected such that with nospeech waves being transmitted, thesensitivities of theseamplifier-detectors are effectively reduced so that they cannot befalsely operated'by noise of 'the maximum amplitude which might beexpected to be received from the circuit TC. This value would be in theorder'of 8-10 decibels. Echoes of signals received in the receivingcircuit RC cannot cause false operation of the'syle labic device 56,because such received signals, or rather the preceding tone waves, willrhave previously operated the receiving control circuit 59 and thus relay65 to block the circuit TC at a point 61 in front of the syllabicdevice. The sensitivities of the transmitting amplifier-detec-` tors 48,49 with the removable losspads 53 and 55 removed from their inputs areinitially ma'de approximately'the same by adjustment ofthe manuallyvariable loss pads'52, 54 with respect to incoming energy, thesensitivity ofthe receiving amplifier-detector 62 in receiver controlcircuit 59 being set by adjustment of loss pad 6I so that it will not befalsely operated by incoming static `or line noise in the absence 'ofspeech transmission.

Part of the voice energy in front of the delay circuit 31 will passintocircuit 50 and will cause the syllabic device 56 to be operated toenergize the relays 51 and 58 in its output. Relay 51 vis designed to bequick-operating, for example, to operate in 0.03 second, but theoperating time of relay 58 is made somewhat longer, the slight delay inoperation providing somev noise protection. Relay 51 is designed to havea small'hang-over time somewhat longer than the operate time, say 0.06second, and the hang-over time of relay 58 is preferably made slightlylonger than that of relay 51, being for example, about 0.09 second.

, Relays 51 and 58 will operate quickly to respectively short-circuitthe loss pad 53 in the input of amplifier-detector 48 and toshort-circuit the loss pad 55 in the input of ampliiierdetector 49thereby eifectively increasing the sensitivities of theseampliiier-detectors by, say, 10 decibels, the sensitivity increase ofthe amplifierdetector 49 because of the longer operating time of relay58 becoming effective about the time the voice Waves reach the output ofdelay circuit 31. 'I'he amplifier-detectors 48 and 49 are now incondition to operate relay 41 quickly by impressed speech waves, andhold it operated longer as these waves decay, there being a hangover ofrespectively 0.06 second and 0.09 second following the release of thesyllabic device 56 on this sensitivity increase. Release of relays 51and 58 by reducing sensitivity of amplierdetectors 48 and 49 preventstheir continued operation on line noise.

The voice waves impressed on the ampliiierdetector 43 through circuit 50will cause its immediate operation to quickly operate the tone controlrelay 41 in its output and start the energization of thedelayed-operating receiving disabler relay 69 throughdelay circuit 10.The voice currents in the signaling path meanwhile are being delayed inthe delay circuit 31, so that the operation of amplifier-detector 49 inresponse to the voice currents through circuit 5|, though itssensitivity has been increased, is delayed with respect to that ofamplifier-detector 48 for a time equal to the time of transmissionthrough delay circuit 31, which is determined as the time necessary toreduce to a tolerable amount the clipping in the speech transmission ofan average telephone subscriber.

The tone control relay 41 is designed to be quick-operating and to haveno hang-over in operation itself so that with the increasedsensitivities of the two amplifier-detectors 48, 49 the relay in its onand off operation will follow the voice waves closely, giving a shorthang-over following the passage of the voice wave through the delaycircuit 31, but not dropping out too frequently during continuoustransmission of speech because of the somewhat tandem action of the twoamplifier-detectors. Thus, the tone energy from source 45 under controlof relay 41 will be applied over the transmitting circuit TC to theradio transmitter, and removed therefrom simultaneously with the riseand fall of the speech energy in the circuit.

The speech currents in the output of delay circuit 3l' will pass throughamplier 38,`hybrid coil H2, bilateral privacy 39, hybrid coil H3 andband-pass filter 40 to the radio transmitter I and will be radiatedthereby, as will be the preceding tone f1.

The operation of the distant terminal (not shown) in response to thereceived tone and speech signals will be explained by referring to theapparatus at that terminal by the same reference numbers as the similarapparatus at the local terminal (Figs. 3 and 4), but followed by a primemark.

The tone waves f1 received by the radio receiver 2 will be detectedtherein and transmitted over the receiving circuit RC. The band-passfilter 4 I will suppress the tone waves from that circuit, but they willbe selected by the parallel band-pass BUin the receiving control circuit59' and will pass through resistance pad 6l to the amplifierdetector 62causing its operation to energize the windings of relays 63', 64', S5and 66' in its output. The singing suppressor relays 63 and 64' willthen operate to remove the normal disabilities at the points 43 and 44in the receiving circuit RC', and the echo suppressor relays 65 and 65will substantially simultaneously operate to disable the transmittingcircuit at the points 61 and 68.

With the tone following speech control closely, the tone-operatedcontrol circuit 59 at the receiving terminal may be given an appreciablehang-over in operation, say 0.05 second, without making the system havean appreciably longer breaking time than present Vodas circuitsemploying appreciable hang-over at both terminals. This hang-over may beapplied readily by making the singing suppressor relays 63 and 64 andthe echo suppressor relays slow-releasing to the required degree.

In order that lock-out time may be reduced in a system employing, ateach terminal, circuits as shown in Figs. 3 and 4, the operation of thereceiving disabler relay 59 at the transmitting end in response to thetransmitted voice currents is delayed with respect to that of thecontrol tone relay 41 for a time interval T substantially equal to theone-way transmission time between the terminal stations, or more exactlyfor a time equal to that required for the tone from the transmittingterminal to pass from the contacts of the tone relay 41 to thetone-operated amplifierdetector 62 at the receiving terminal station.For the reasons given above in connection with Fig. 2, this will limitthe lock-out period to a small value, say, in the order of 5milliseconds for a typical system. Singing may not occur because thelocal receiving circuit RC is in its normally disabled condition, andwill be prevented from becoming enabled due to the delayed operation ofrelay 69 before the distant receiving circuit RC is enabled.

As in the case of the system shown in Fig. 2,

the receiving disabler relay (50, 58') at the terminal stations ispreferably given a hang-over in operation beyond the release of the tonecontrol relay (41, 41') slightly more than T, say, T -l- .05 second, toreduce the effect of possible false operation of the tone-operatedamplifier detector.

When the voice signals reach the receiving terminal they will be pickedup and detected inthe radio receiver 2 and then transmitted over thereceiving circuit through vband-pass lter 4|', bilateral privacy 59,amplifier 42', hybridy coil HV' and telephone line TL' to the listeningsubscriber, the receiving circuit RC having been previously enabled atthe points 43 and 44' by the tone waves in the manner which has beendescribed.

'When the transmission of voice signals ceases at the transmitterstation, the syllabic device 56 releases and causes, after the slighthang-over period referred to above, the relays 51 and 58 to release toreinsert the loss pads 53 and 55 inthe input ofthe amplier-detectors 48and- 49 to reduce their sensitivities. The release of amplifierdetectors48 and 49 with cessation in the applied speech waves causes the tonecontrol relay to immediately release to cut off the supply of controltone to the radio transmitter I. The receiving disabler relay after themuch longer hangover interval referred to above releases to make thereceiving control circuit 59' operative again. At the receivingterminal, the release of the amplier-detector 62' inthe receivingcontrol'circuit 59' with cessation in the supply of control tone theretowill cause the release of relays 63', 64', 65 and 66 at the end of theirhang-over periods specified above to return the transmitting circuit TC'to its normally operative condition and to redisable the receivingcircuit RC at the point 43 and 44.

It is to be understood that the values specied above for the operate andrelease or hang-over times of the switching relays in the system of theinvention described above are to be taken by way of example only and notas limiting the invention.

Various modifications of the circuit illustrated and described withinthe spirit and scope of the invention will be apparent to personsskilled in the art.

What is claimed is:

1. A two-way signal transmission system comprising terminal stations anda two-way signal transmission medium connecting said stations, eachstation including a normally enabled signal transmitting circuit, asignal receiving circuit, switching apparatus for rendering saidreceiving circuit operative and disabling said transmitting circuit tosuppress echoes and prevent singing, selectively operated by distinctivecontrol waves transmitted over said medium. from the other terminalunder control of the signals transmitted therefrom, and means forimproving the operation of said switching apparatus comprising means formaking the transmitted control waves follow the transmitted signalsclosely in on and oi operation, and means providing the controlwave-operated switching apparatus at the terminals with suicienthang-over in operation to aid in the operation through fading of thecontrol waves and to insure suppression of all signal echoes withcessation in the applied control waves.

2. A two-Way signal transmissionsystem comprising terminal stations anda two-wayl signal transmission medium connecting said stations, eachstation including a normally enabled signal transmitting circuit, anormally disabledsignal receiving circuit, switching apparatus forrendering said receiving circuit operative and disabling saidtransmitting circuit to suppress echoes and prevent singing, selectivelycontrolled by control waves of distinctive frequency transmitted oversaid medium from the other terminal in response to initiation of signalwaves thereat for transmission, and means to reduce signal transmissionlockout and clipping, and to facilitate breaking, comprising means ateach terminal for making the transmitted control waves follow thetransmitted signals closely in on and off operation, means providingsaid switching apparatus at each terminal with sufcient hang-over inoperation to insure suppression of all signal echoes when the appliedcontrol Wave energy falls off, and means at each terminal alsoresponsive to the initiation of signal waves thereat to disable thecontrol wave-operated switching apparatus at the ter- .minal but with atime delay with respect to the transmission ofthe control wavessubstantially equal to the transmission time between the two terminalsof the system, and to maintain the switching apparatus so disabled for alike time interval after the transmission of said control waves ceases.

3. In a, two-way signal transmission system including two stationsconnected by a two-way transmission medium, a normally enabled signaltransmitting circuit and a normally disabled signal receiving circuit ateach station, control means at one station responsive to the initiationof signals thereat for transmission for sending distinctive controlwaves out over said medium to condition the other station for signalreceiving only, and responsive substantially immediately to cessation inthe transmitted signalsI to stop lthe transmission of said controlwaves, and means to maintain said other station so conditioned for asufficient time interval `after cessation in the received control wavesto suppress echoes of the received signals. i

4. In a two-way signal transmission-system including two stationsconnectedlbyv a two-way transmission medium, means at each stationresponsive to the initiation of signals thereat for transmission to theother station, to send out control waves of distinctive frequencydiffering for each station, over said medium to the other station, andresponsive substantially` immediately to cessation in transmittedsignals to stop the transmission of said control waves, switching meansat veach station selectively responsive to received control waves tocondition the station for signal receiving only, means at each stationfor maintaining the station so conditioned for a suflicient timeinterval after cessation in receivedv control waves to suppress allechoes of the received signals, and other switching means at eachstation also responsive to the signals initiated thereat fortransmission, but operative at a time after the control waves have beensent out from the station substantially equal to the transmission timebetween the stations, to disable the control wave-responsive switchingmeans at the lsame station, 'and to lmaintain it disabled for a liketime interval after control waves transmission from the station ceases.

5. A two-way signal transmission system. comprising two stations and atwo-way transmission medium connecting .said stations, each stationcomprising Va transmitting vcircuit which is enabled and a receivingcircuit which is disabled in the absence of signal transmission betweenthe stations, control means operatively responsive to the initiation ofsignals forv transmission to send out control waves of distinctivefrequency, differing for each station, over said medium to the otherstation, and responsive substantially immediately to cessation in thetransmitted signals to stop the transmission of the control wavesl fromthe station, switching means selectively responsive to the receivedcontrol waves to disable the transmitting circuit and render thereceiving circuit of the station operative, and other switching meansalso operatively responsive to the signals initiated at the station fortransmission but at a time after said control means operatessubstantially equal to the transmission time between said stations, tordisable the selective switching means at the station, and to maintainsaid selective switching means disabled for an equivalent time intervalafter control wave transmission from the station ceases.

6. A two-way telephone transmission system comprising a two-waytransmission medium connecting stations each including a normallyenabled transmitting circuit, a normally disabled receiving circuit,contro-l means responsive to initiation of telephone signals in saidtransmitting circuit to cause distinctive control waves to be sent outover said medium to the other station ahead of the signals, andresponsive substantiallyimmediately to any cessation in the transmittedsignals to stop the transmission of said control waves, switching meansselectively responsive to the distinctive control waves received oversaid medium from said other station to disable said transmittingcircuit, and to render said receiving circuit operative to receive thefollowing signals, said switching means having an appreciable hang-overin operation so as to maintain the transmitting circuit disabled for asuicient time to suppress all signal echoes when the applied controlwaves cease and other slowoperating and slow-releasing switching meansalso responsive to the signals initiated in said transmitting circuit tocause the disablement of the selective switching means at the stationsubstantially at the time the control Waves transmitted from the stationinitiate operation of the selective switching means at the otherstation.

7. The telephone transmission system of claim 6 in which said controlmeans comprises a source of said distinctive control waves, a delaycircuit in the transmitting circuit of the station, two Wave-responsivedevices connected to said transmitting circuit on either side: of saiddelay circuit, the sensitivities of said wave-responsive devices beingnormally set so as to prevent their operation by a predetermined amountof circuit noise received from said transmitting circuit, a switchingdevice responsive quickly to operation of either of said Wave-responsivedevices to connect said source to said medium, a syllabic deviceconnected to said transmitting circuit, substantially unresponsive tonoise but responsive to telephone signals transmitted over said circuit,to so increase the sensitivities of said wave-responsive devices as toenable their quick operation by the transmitted signals, said switchingdevice having substantially no hang-over in operation so that it willrelease quickly to disconnect said source from said medium in responseto the release of both wave-responsive devices when the applied signalinput thereto ceases, the time delay of said delay circuit beingdetermined by the tolerable clipping experienced in speech transmissionof an average telephone subscriber.

8. A terminal station for a two-way signal transmission medium,comprising a normally enabled transmitting path including a Vogad, 'anormally disabled receiving path, switching means connected to thetransmitting path and responsive to initiation of signals therein tocause distinctive control energy to be sent out over said medium inadvance of the signals, the disabllement of said receiving path beingproduced by a variable loss device therein whose loss is a function ofthe gain of the Vogad in the transmitting path, switching meansconnected to said receiving path in front of the disabling pointtherein, selectively responsive to distinctive control waves receivedfrom said medium to remove the disabling loss from the receiving pathand to disable said transmitting path in front of said Vogad and saidswitching means connected to that path, and means responsive tooperation of the latter switching means to disable the selectiveswitching means connected'to the receiving path.

9. In combination, a one-way telephone wave transmission path subject tointerfering noise waves, a transmission delay circuit in said path, aseparate wave-responsive device connected to said path on either side ofsaid delay circuit, the sensitivity of each Wave-responsive device beinginitially set so as to prevent operation of the device by noise wavesimpressed thereon from said path of a given maximum amplitude, a.-syllabic device connected to said path in front o f` said delay circuit,unresponsive to noise waves but adapted to be operated in response tothe syllabic variations in applied telephone waves to increase thesensitivity of both of the waveresponsive devices to allow their quickoperation in response to telephonie Waves applied from each path, and aquick-operating, quick-releasing switching device operatively responsiveto operation of either of said wave-responsive devices.` the delayv timeAselected for said delay circuit being determined by the clipping of thevoice transmission of an average telephone subscriber.

CHARLES C. TAYLOR.

